Structural and functional definition of the human chitinase chitin-binding domain

Area of Science:

• Biochemistry
• Molecular Biology
• Immunology

Background:

• Mammalian chitinase is a macrophage-expressed enzyme found in atherosclerotic plaques.
• Elevated chitinase levels are observed in Aspergillus-infected guinea pigs, but its physiological role remains unclear.

Purpose of the Study:

• To characterize the chitin-binding domain of mammalian chitinase.
• To understand the enzyme's interaction with its insoluble substrate, chitin.

Main Methods:

• Site-directed mutagenesis to identify the minimal chitin-binding sequence (C-terminal 49 amino acids).
• Analysis of recombinant chitin-binding domain for disulfide linkages and substrate specificity.
• Use of fluorescently tagged chitin-binding domain to assess binding to fungal species.

Main Results:

• The C-terminal 49 amino acids constitute the minimal chitin-binding domain, essential for insoluble chitin hydrolysis but not soluble substrate hydrolysis.
• Six cysteine residues within this domain form three disulfide linkages, critical for chitin binding.
• The domain specifically binds chitin, not other polysaccharides like cellulose or chitosan.
• Demonstrated chitin-specific binding to fungal cell walls of Saccharomyces cerevisiae, Candida albicans, Mucor rouxii, and Neurospora crassa.

Conclusions:

• The minimal chitin-binding domain possesses specific structural features (disulfide bonds) required for binding and hydrolyzing insoluble chitin.
• This domain exhibits specific binding to fungal cell walls, suggesting a role in host-pathogen interactions or immune responses involving chitin.